getopt - parameters that aren't flags?

Question

I'm trying to use the getopt function for the first time only I'm having problems with arguments that aren't flags. For instance, in my code when a unknown argument is given I want to use it as a input file. When I run this with only a file name it is not printed, if I first use a flag, any flag, then I can print it.

How can I fix this?

Thanks

 #include <stdio.h>
 #include <getopt.h>

 static struct option long_options[] = {
  {"help",  no_argument,       0, 'h'},
  {"input",  required_argument, 0, 'i'},
  {"output",  required_argument, 0, 'o'},
  {"algorithm",  required_argument, 0, 'a'},
  {0, 0, 0, 0}
 };

 int main(int argc, char *argv[]) {
  int c;
  int option_index = 0;

  while(42) {
   c = getopt_long(argc, argv, "hi:o:a:", long_options, 
     &option_index);
   if(c == -1)
    break;

   switch(c) {
    case 'h': /* --help */
     printf("--help flag\n");
     break;
    case 'i': /* --input */
     printf("--input flag\n");
     break;
    case 'o': /* --output */
     printf("--output flag\n");
     break;
    case 'a': /* --algorithm */
     printf("--algorithm flag \n");
     break;
    default: /* ??? */
     fprintf(stderr, "Invalid option");
     return 1;
   }

   if(optind < argc) {
    printf("other arguments: ");

    while(optind < argc) {
     printf ("%s ", argv[optind]);
     optind++;
    }

    printf("\n");
   }
  }

  return 0;
 }

Answer 1

The loop should only contain the switch. Process the residual arguments in a separate (not nested) loop:

#include <stdio.h>
#include <getopt.h>

static struct option long_options[] =
{
    {"help",  no_argument,       0, 'h'},
    {"input",  required_argument, 0, 'i'},
    {"output",  required_argument, 0, 'o'},
    {"algorithm",  required_argument, 0, 'a'},
    {0, 0, 0, 0}
};

int main(int argc, char *argv[])
{
    int opt;
    int option_index = 0;
    int i;

    while ((opt = getopt_long(argc, argv, "hi:o:a:", long_options, &option_index)) != -1)
    {
        switch(opt)
        {
            case 'h': /* --help */
                printf("--help flag\n");
                break;
            case 'i': /* --input */
                printf("--input flag (%s)\n", optarg);
                break;
            case 'o': /* --output */
                printf("--output flag (%s)\n", optarg);
                break;
            case 'a': /* --algorithm */
                printf("--algorithm flag (%s)\n", optarg);
                break;
            default: /* ??? */
                fprintf(stderr, "Invalid option %c\n", opt);
                return 1;
        }
    }

    for (i = optind; i < argc; i++)
        printf("Process: %s\n", argv[i]);

    return 0;
}

There is a way to have GNU getopt() and getopt_long() return file name arguments as if they were options with the 'letter' ^A '\1'; use '-' as the first character of the short options string, and trap '\1' in the switch; the value of optarg is the name of the file.

#include <stdio.h>
#include <getopt.h>

static struct option long_options[] =
{
    {"help",  no_argument,       0, 'h'},
    {"input",  required_argument, 0, 'i'},
    {"output",  required_argument, 0, 'o'},
    {"algorithm",  required_argument, 0, 'a'},
    {0, 0, 0, 0}
};

int main(int argc, char *argv[])
{
    int opt;
    int option_index = 0;
    int i;

    while ((opt = getopt_long(argc, argv, "-hi:o:a:", long_options, &option_index)) != -1)
    {
        switch(opt)
        {
            case 'h': /* --help */
                printf("--help flag\n");
                break;
            case 'i': /* --input */
                printf("--input flag (%s)\n", optarg);
                break;
            case 'o': /* --output */
                printf("--output flag (%s)\n", optarg);
                break;
            case 'a': /* --algorithm */
                printf("--algorithm flag (%s)\n", optarg);
                break;
            case '\1':
                printf("File: %s\n", optarg);
                break;
            default: /* ??? */
                fprintf(stderr, "Invalid option %c\n", opt);
                return 1;
        }
    }

    for (i = optind; i < argc; i++)
        printf("Process: %s\n", argv[i]);

    return 0;
}

However, you do need the loop after the argument processing loop in case your cantankerous user types:

program -- abc def

The '--' terminates the while() loop without processing the file name arguments.

Answer 2

It doesn't work because you break out of the outer while loop when getopt_long returns -1 (which indicates that there are no more options).

You need to move the if (optind < argc) block out of the outer while loop; it doesn't belong there anyway. This might be clearer if you wrote that outer while loop as:

while ((c = getopt_long(...)) != -1)
{
    switch (c)
    {
        /* Deal with flags. */
    }
}